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The interaction between adjacent active sites is crucial to balance the efficiency and utilization of functional atoms in single-atom catalysts. Herein, the catalytic activity of hydrogen evolution reaction at different site (nitrogen coordinated transition metal centers embedded in graphene) distances was comprehensively investigated by density functional theory calculations. The results show that a proximity effect of reactivity and site spacing can be identified in the Co-series single-atom catalysts. Although the proximity effect is more linearly responded with the site spacing along x direction, an optimal distance of ∼0.8 and ∼2.8 nm are found for Co and Rh, Ir atoms, respectively. An in-depth analysis of the electronic property reveals that the proximity effect is caused by the distinct net charge of the active site, which is affected by the d z 2 position relative to EF. Subsequently, an excess electron nodal channel in x direction was found to serve as a communication pathway between the active sites. Through the finding in this work, an optimal Fe-N2C2 structure was deliberately designed and has shown prominent proximity effect as Co-series do. The results reported in this work provide a simple and effective tuning method for the reactivity of a single-atom catalyst. [ABSTRACT FROM AUTHOR]

Molybdenum carbides (MoC) are regarded as promising candidates for electrocatalytic hydrogen evolution reaction (HER) as their stabilities, high conductivities. Non-metallic doping is a robust way to enhance the HER activity of MoC in experiments, yet the systematic theoretical study is still lacking. In this work, we investigate the surface doping effect on HER activity of C-terminated γ -MoC(100) by density functional theory (DFT). The thermodynamical stability and realistic catalytic surface of doped surfaces, including mono- and co-doping by three elements (N, P and S) with various doping ratios, are verified by formation energies and surface Pourbaix diagrams, respectively. According to the hydrogen adsorption ability on different coverage and the calculated exchange current densities (i 0) of the doped surfaces, the surfaces doping in range of (P% > 60% and N% > 5%), (60% < N% <85% and P% < 25%), and (60% < N% < 85% and S% < 25%), show larger i 0 (i 0 > 4 mA/cm2). Especially the N/P co-doping γ -MoC(100), their larger i 0 in greater range enables their promising excellent performance in hydrogen evolution in experiments. The improved HER activities of doped MoC(100) are ascribed to suitable hydrogen adsorption abilities tuned by suitable p z -band centers and the charge redistribution. Our DFT simulations provide more insight and guidance for improving the HER performance of electrode catalysts using non-metallic doping effects. [Display omitted] • N/P co-doped MoC(100) at any ratio shows high exchange current density for HER. • N and P doping will downshift p z -band center and tune hydrogen adsorption ability. • The charge redistribution domains the oscillation character of Δ G H in P-doped surfaces. [ABSTRACT FROM AUTHOR]

Could materials only including non-noble metals be good HER catalysts? To deal with this puzzle, computational screening of 132 different non-noble transition metal A/B surfaces for HER (A monolayer on B), are carried out by first-principles calculations systematically. The formation energies and dissolution potentials are calculated to access stabilities of A/B in vacuum and in solution, respectively. The realistic catalytic surfaces with oxygen or hydroxyl (co)adsorption are confirmed by analyzing Surface Pourbaix diagrams. Finally, three A/B surfaces (Cu/Mo, Mo/W, and Ti/Nb) with high stabilities and high HER exchange current densities (8.51, 3.39, 2.83 mA/cm2) are screened out. Further electronic properties analysis reveals that the different interactions between H 1 s and d orbital, s orbital of A atoms will determine the feasible application of d and s band centers to uncover water effect and substrate effect on hydrogen adsorption ability, and tune the HER activity of A/B in the end. • Several non-noble transition metal A/B surfaces with high HER performance are screened out. • Surface Pourbaix diagram, thermodynamical stability and dissolution behaviors are explored. • d - (s -) band centers are applied to uncover water effect and substrate effect on Δ G H. [ABSTRACT FROM AUTHOR]

Photoelectric catalytic and solar cells are two effective ways to solve the global energy shortage and environmental pollution problems. However, low carrier separation efficiency has been becoming a common problem of current photocatalytic water decomposition and solar cells. In this work, both the electronic structures and optical properties of Janus MoSSe/MoGeSiN 4 vdW heterostructures were systematically studied by density functional theory. The results show that the Janus MoSSe/MoGeSiN 4 vdW heterostructure with Se/Ge interfacial contact (Se/Ge heterostructure) is a direct band gap semiconductor. Interestingly, for the Se/Ge heterostructure, spatial separation of the photo-generated electrons and holes is expected, due to the conduction band minimum (CBM) and the valence band maximum (VBM) separately locating on the MoGeSiN 4 and MoSSe layer. Besides, the Se/Ge heterostructure not only exhibits considerable absorption index in the visible light range but also maximum theoretical photoelectric conversion efficiency approaches 26.4 %, which can be furthermore enhanced by varying the layer distance and biaxial strain. The Se/Ge heterostructure shows high carrier mobility, obvious carrier separation, notably visible light absorption and tunable photoelectric properties, making it promising candidates for novel two dimensional photocatalysis devices and solar cells. • MoGeSiN 4 /MoSSe heterostructure can form type-II band alignment with obvious carrier space separation. • Has excellent visible light absorption and high electron mobility of 6048.43 cm2V−1S−1. • The photoelectric conversion efficiency of heterostructure can be enhanced by strain and layer distance. [ABSTRACT FROM AUTHOR]

[Display omitted] • 1,3,6,8-BCZ could be efficiently degraded in S-ZVI/PDS system. • Both radical and nonradical pathways play roles in the degradation of 1,3,6,8-BCZ. • •OH and 1O 2 are the dominant reactive oxygen species (ROS). • 1,3,6,8-BCZ could be degraded by 1O 2 via 1,4-cycloaddition. • O 2 •− plays a synergistic role in the intermediate degradation and 1O 2 generation. Polyhalogenated carbazoles (PHCZs) are a kind of emerging contaminants which have widespread distribution, strong potential persistence, high bioaccumulation, and dioxin-like toxicity. In this study, we used sulfidated zero-valent iron (S-ZVI) synthesized by ball-milling to activate peroxydisulfate (PDS) for the degradation of 1,3,6,8-tetrabromocarbazole (1,3,6,8-BCZ). The initial 1 mg/L 1,3,6,8-BCZ was degraded by 92.9 % within 30 min in S-ZVI/PDS system. The pseudo-first-order rate constant was estimated as high as 0.182 min−1. Electron paramagnetic resonance (EPR) study and quenching experiments results revealed that •OH, 1O 2 , O 2 •−, and SO 4 •− played roles in the degradation of 1,3,6,8-BCZ, with their contribution percentages of 32.8 %, 32.5 %, 24.8 % and 9.9 %, respectively. By further density functional theory (DFT) calculations, radical addition by •OH and 1,4-cycloaddition by 1O 2 were suggested as the dominant channels for 1,3,6,8-BCZ degradation, while O 2 •− exerted an indirect synergistic influence. Based on the identification of intermediates and analysis of Fukui indices, 1,3,6,8-BCZ degraded through three degradation pathways, namely hydroxylation, debromination, and C–N bond cleavage. The S/Fe molar ratio, S-ZVI dosage, PDS dosage, reaction temperature, initial pH, and water matrices had influences on the degradation in 1,3,6,8-BCZ, result in a great toxicity reduction. Overall, this research developed an effective and feasible approach for the treatment of PHCZs, and which gave deep insights into the vital roles and synergistic effects of various reactive species based on the combination of experimental study and theoretical calculation. [ABSTRACT FROM AUTHOR]